The present invention relates to a power semiconductor device, and more particularly to a power semiconductor device constructed by a field effect transistor.
For the purpose of use as a power semiconductor device a vertical field effect transistor, and more specifically a DMOS transistor of vertical double-diffusion structure in which channel length is defined by diffusions for forming base and source regions and is, hereinafter, used under the above-mentioned meanings is considered promising.
Taking the case of N channel as an example, a DMOS transistor is formed in a semiconductor chip that has an N.sup.- type epitaxial layer formed on an N.sup.+ type semiconductor layer. In the epitaxial layer on the surface part of the semiconductor chip there is formed a P type base region, and an N.sup.+ type source region is formed within the base region. Of the N.sup.- type epitaxial layer of the semiconductor chip the part where the base region is not formed is the drain region. A gate insulating film is formed on the base region in between the source region and the drain region, and a gate electrode is formed on the gate insulating electrode.
Actually, the gate electrode is arranged on the gate insulating film in a meshlike form. The source region is formed in a manner self-aligned with the gate electrode. Namely, a large number of DMOS cells are built in parallel in the semiconductor chip.
When the source electrode (an electrode connected to the source region) is grounded, the drain electrode (an electrode on the rear surface of the semiconductor chip) is biased positively and a positive voltage is applied to the gate electrode, an N type inversion layer (channel) is formed on the surface part of the base region below the gate electrode, and a current flows from the drain electrode through the N.sup.+ type semiconductor layer, the N.sup.- type epitaxial layer, the N type inversion layer and the source region. Accordingly, by connecting a load between a power supply and the drain electrode it becomes possible to drive the load. Further, by reducing the voltage applied to the gate electrode it is possible to interrupt the current and stop the driving of the load, so that the load can also be given the function of a semiconductor switch.
The driving current of such a power semiconductor device is set at a desired level in the range of 1 to 10 amperes. Because of the flow of such a high current in a power semiconductor device there exists a possibility of giving damage to the power semiconductor device due to overheating. In the past, prevention of such a thermal breakdown required a damage preventive mechanism of complex structure with an extremely large area for the semiconductor chip which was an obstruction to putting it to practical use. For the prevention of the thermal breakdown it is important as a first step to be able to detect the temperature of the chip.